1. Technical Field
The present invention relates to a switching device that switches between electric conduction and electric cutoff by bringing or separating a movable member into contact with or from a contact of a fixed member.
2. Related Art
Conventionally a switch, which controls the electric conduction and the electric cutoff by bringing or separating the movable member into contact with or from the fixed member, is in widespread use. In the switch, an arc (a discharge phenomenon in which a current moves in a gap between a surface of the movable contact and a surface of the fixed contact) is generated mainly at moment at which the movable contact is separated from the fixed contact. Conventionally, a lifetime of the switch is shortened because a quality of material of the surface degrades by the generation of the arc.
Thus, there has been proposed a technology, in which a magnetic field is generated in a direction orthogonal to a current moving direction and a force is caused to act on electrons constituting the arc to encourage diffusion of the arc. For example, Japanese Unexamined Patent Publication No. 57-84520 discloses the switch in which the magnet generating the magnetic field at a point at which the fixed contact and the movable contact come into contact with each other is provided at a fixed position in a direction intersecting the movable contact moving direction. Japanese Unexamined Patent Publication No. 2004-178953 discloses a magnetic arc elimination type switch device including the magnet that applies the magnetic field to the arc with magnetic flux density of 85 mT or more.
In the conventional technologies disclosed in Japanese Unexamined Patent Publication No. 57-84520 and 2004-178953, it is assumed that the magnetic field is generated using the metallic magnet. On the other hand, it is conceivable that a plastic magnet is used instead of the metallic magnet for the purpose of cost reduction or weight reduction of the switch. However, the use of the plastic magnet may result in the surface of the plastic magnet being melted by the heat of the arc. This is attributed to the fact that a temperature of the heat generated by the arc is higher than a melting point of plastic while being lower than a melting point of metal.
Degradation of switch performance due to the melt of the surface of the plastic magnet will be described with reference to FIG. 10. FIG. 10 is a graph comparing time (cutoff time) necessary for the switch including the metallic magnet to cut off the current and time necessary for the switch including the plastic magnet to cut off the current before and after a test to switch the switch from a conduction state to a cutoff state one hundred thousand times when a time constant is set to 7 ms under an inductive load of 125 volts and 6 amperes.
As illustrated in FIG. 10, because the surface of the metallic magnet is not melted, the cutoff time does not change before and after the test. On the other hand, since the surface of the conventional plastic magnet is melted, an average value of the cutoff time increases prominently after the test, and a variance of the cutoff time also increases significantly. This is because a magnetic force is weakened by the melt of the surface to degrade the performance diffusing the arc. Therefore, the switch performance also degrades.
In the case where the magnet is disposed while brought close to the position at which the arc is generated for the purpose of downsizing of the switch, or in the case where the switch is used to cut off the current of a higher load, melting may be worse, because the magnet is strongly influenced by the heat of the arc. In view of the above, the use of the metallic magnet as the magnet diffusing the arc exists as a natural assumption commonly recognized by those skilled in the art. Therefore, those skilled in the art hardly consider a configuration in which the plastic magnet is used.